The present invention relates to a concentrated dampening water useful for offset printing process and also a process for using it.
The lithography is a printing method wherein the essential immiscibility between water and an oil is utilized. The printing plate surface has a region which receives water but repels an oily ink and another region which repels water but receives the oily ink. The former forms a non-image area and the latter forms an image area. A desensitizer has such an effect that when the non-image area is wet with a dampening water containing it, the ink-repellency of the non-image area and ink-receptivity of the image area are increased, because the interfacial chemical difference is enlarged between the image area and the non-image area.
Well-known desensitizers include aqueous solutions containing an alkali metal dichromate, ammonium dichromate, phosphoric acid or its salt such as ammonium phosphate, or a colloidal substance such as acacia gum or carboxymethylcellulose (CMC).
However, the dampening water containing such a desensitizer has a defect that it cannot easily and homogeneously wet the non-printing area of the plate, staining the prints and that the control of the feeding amount of the dampening water to printing plate necessitates a considerably delicate technique.
To overcome this defect, Dahlgren dampening system has been proposed wherein about 20 to 25% aqueous solution of isopropyl alcohol is used as a dampening water. This method has various advantages in the workability and accuracy of the prints such as that the wetting of the non-image area is improved, that the amount of the dampening water is reduced, that the balance between the amounts of the printing ink to be fed and water to be fed can be easily adjusted, that the quantity of the dampening water to be emulsified into the printing ink is reduced, and that the transfer of the printing ink to the blanket is improved.
However, since isopropyl alcohol easily evaporates, a specific apparatus is necessitated for keeping the isopropyl alcohol concentration of the dampening water constant, which elevates the cost. Further, isopropyl alcohol is not preferred from the viewpoint of the working environment, because it has peculiar, bad smell and toxicity.
Another problem is that even when the dampening water containing isopropyl alcohol is used for an ordinary offset printing method in which a dampening roller is used, its effect cannot be obtained, since isopropyl alcohol evaporates on the roller and the plate surface.
Further, the social concern about environmental polluiton is increasing, chromium ion concentration of waste water is severely controlled, and the use of organic solvents such as isopropyl alcohol is going to be regulated from the viewpoint of the hygienic safety. Under these circumstances, the development of a desensitizer free from these organic solvents has been demanded.
To attain the object, various compositions containing a surfactant are described in, for example, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as `J. P. KOKOKU`) Nos. 55-25075, 55-19757 and 58-5797. However, when such a composition is used as a dampening water, a considerably high surfactant concentration of the desensitizer is necessitated in order to obtain a surface tension of 35 to 50 dyne/cm. In the practical lithography, the ink and water vigorously move under the conditions of the ink roll, the printing plate and the dampening water-feeding roll, which rotate at a high speed. Therefore, problems such as that water adheres to the ink film and that the ink is diffused on the water surface, are posed. However, the above-described combination of the surfactants is insufficient for completely solving these problems. Another defect of the dampening water containing such a surfactant is that it easily bubbles during the transportation through pumps or by stirring.
In U.S. Pat. No. 3,877,372, is described a solution containing a mixture of ethylene glycol monobutyl ether with at least one of hexylene glycol and ethylene glycol. In U.S. Pat. No. 4,278,467, is described a dampening water containing at least one of 2-hexyloxyethanol, diethyleneglycol n-hexylether, 2-ethyl-1,3-hexanediol, n-butoxyethylene glycol acetate, n-butoxydiethylene glycol acetate and 3-butoxy-2-propanol. In Japanese Patent Publication for Opposition Purpose (hereinafter referred to as `J. P. KOKOKU`) No. 57-199693(=U.S. Pat. No. 4,560,410), is described a dampening water containing 2-ethyl-1,3-hexanediol and at least one of completely water-soluble propylene glycol, ethylene glycol, dipropylene glycol, diethylene glycol, hexylene glycol, triethylene glycol, tetraethylene glycol, tripropane glycol and 1,5-pentanediol. These dampening water compositions are advantageous from the viewpoints of safety and hygiene, since they are free from isopropyl alcohol. However, they still have problems that when a PS plate having an anodized aluminum support is used, the wetting of the non-image area during the printing is insufficient and, in particular, the non-image area is stained during high-speed printing and the shapes in the halftone dot-image area become abnormal, enlarged and uneven, that is, so-called plugging of half-tone dots image is caused in the halftone dot-image area. Another problem is that the solubility of 2-ethyl-1,3-hexanediol in water is insufficient and this compound is unsuitable for the preparation of a dampening water concentrate of a high concentration or an additive for the dampening water.